Pneumatic brake actuators form part of the pneumatic braking system of commercial vehicles having a large gross vehicle weight, including trucks, buses and trailers requiring a braking system which responds rapidly with substantial braking power. A typical diaphragm-type pneumatic brake actuator includes a housing having cup-shaped housing members including opposed rim or flange portions, a flexible diaphragm which is cup-shaped in its relaxed condition including a central portion, a generally conical side wall which surrounds the central portion and a generally radial rim portion which extends between the rim portions of the housing members. A brake actuator further includes a piston having a contact surface which engages the central portion of the diaphragm and which reciprocates with the central and side wall portions of the diaphragm in response to pneumatic pressure changes on opposed sides of the diaphragm from a first position, wherein the diaphragm is extended to a cup-shape, to a second inverted position to actuate the vehicle braking system. The brake actuator is connected by pneumatic lines to the pneumatic braking system of the vehicle to actuate the brake actuator. The piston is operably connected to the braking system of the vehicle to actuate the vehicle brakes.
The brake actuator system includes an emergency or spring chamber having a power spring which actuates the braking system of the vehicle when the pneumatic pressure of the vehicle falls below a predetermined minimum or the parking brake is actuated by the vehicle operator. In a spring brake chamber, a power spring is located in the housing between the end wall and the piston. During normal operation of the vehicle, the pneumatic pressure from the vehicle is received in the power spring chamber on the side of the cup-shaped diaphragm opposite the power spring and piston, thereby normally compressing the power spring. When the pneumatic pressure in the spring chamber falls below a predetermined minimum, the power spring expands and actuates the braking system of the vehicle.
The spring and service chambers may be combined in a “piggyback” assembly as disclosed, for example, in U.S. Pat. No. 4,960,036 assigned to the assignee of this application, wherein the assembly includes a central generally H-shaped flange case and the opposed ends of the flange case are enclosed by cup-shaped housing members to define a service chamber on one side of the flange case and a power spring chamber on the opposed side of the flange case. A central opening in the web portion of the flange case receives a pushrod having a head portion biased against the central portion of the diaphragm in the service chamber opposite the piston and power spring, such that the pushrod is driven against the piston in the service chamber to actuate the vehicle braking system when the pressure in the spring chamber falls below a predetermined minimum pressure. Alternatively, the spring and service chambers may be utilized as separate components of the brake actuator system as is known in the art.
FIG. 1 of the U.S. Pat. No. 4,960,036 illustrates the spring chamber of a conventional dual diaphragm or piggyback pneumatic brake actuator. The assembly includes a generally H-shaped flange case having a central web portion, an outer wall and a radially extending flange. The spring chamber is enclosed by a cover or head having an end wall, a side wall and a flange or skirt portion. The flange portion includes a generally radially extending portion, an axially extending portion and a radially inwardly extending lip which is inelastically deformed as discussed further below. The spring chamber further includes a piston having a central portion and an annular contact portion having an annular contact surface which normally engages the central portion of the diaphragm. The spring chamber further includes a powerful coiled power spring. A power spring and piston guide centers the power spring in the pneumatic chamber and the guide includes a rolled opening which centers dome-shaped end of the piston during operation of the brake actuator as further described below. The spring chamber further includes a pushrod which reciprocates through an opening in the web portion of the flange case as described below. The opening includes annular seals (not shown) which prevent leakage between the pneumatic chambers.
The operation of the pneumatic brake actuator is best shown in FIG. 1 of the U.S. Pat. No. 4,960,036. Pneumatic pressure or gas is received through opening in the side wall of the flange case, pressurizing pneumatic chamber. The gas pressure in pneumatic chamber acts against the flexible diaphragm, compressing the coiled power spring and driving the piston upwardly to be received in the power spring and piston guide. The flexible diaphragm is then cup-shaped and the side wall is frusto-conical or generally conical.
When the pressure in the pneumatic chamber falls below a predetermined minimum, which may occur as a result of a failure of the pneumatic braking system of the vehicle, the power spring expands, driving the pushrod through the opening in the web portion of the flange case, actuating the vehicle braking system. The chamber further serves as a parking brake when the vehicle motor is turned off and the parking brake is actuated by the operator.
A problem with any prior art diaphragm-type brake actuator is formation of a corrosion cell, which negatively impacts lifespan of the brake actuators of the kind. The inside surface of the cover or head of the brake actuator includes a protective coating formed from a powder or autophoretic composition. As the spring contacts the spring guide and/or the cover or when the cover contacts the spring guide, the protective coating is worn off and a galvanic corrosion cell is form between the spring and the cover, the spring and the spring guide, and the spring guide and the cover.
By way of background, the environment for many structures fabricated from metals of different types provides conditions favoring formation of natural corrosion cells. Formation of the corrosion cell results from the galvanic corrosion, which is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal and both metals are immersed in an electrolyte.
When two or more different sorts of metals come into contact in the presence of the electrolyte, a galvanic couple is established because different metals have different electrode potentials, wherein each metal acts as either a cathode or an anode. The electrolyte provides means for ion migration whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal corroding more quickly thereby resulting in formation of the corrosion cell. The presence of electrolyte and conducting path between the metals cause corrosion where otherwise neither metal, if used alone, would have been corroded.
There has, therefore, been a longstanding need to reduce frictional wear between the spring and the head of the brake actuator to increase the life of brake actuators and to eliminate formation of the corrosion cell therebetween.
Another longstanding need is to increase performance of the brake actuator and to provide improved braking performance without increasing the size of the brake actuator.
Still another longstanding need exists for an improved design of the brake actuator that will prevent formation of the corrosion cell and to improve centering of the spring inside the head thereby increasing the life of the brake actuator. These and other problems have been solved by the improved brake actuator described below.